This invention relates to gas-filled bushings for twopressure circuit breakers, and more specifically relates to a novel arrangement for preventing liquefaction of the dielectric gas in a gas-filled bushing and in the low-pressure region of the circuit breaker at low temperatures and for ensuring the dryness of the gas in a gas-filled bushing when used in a two-pressure system.
Gas-filled high voltage bushings are well known in the electrical power art. A typical high-voltage gas-filled bushing is shown in U.S. Pat. No. 3,566,001 in the name of McCloud, issued Feb. 23, 1971 and entitled GAS FILLED BUSHING WITH SPRING BIAS CLAMPING AND INTERNAL FLEXIBLE SHUNT. Bushings of this type are used for high voltage two-pressure gas blast circuit breakers of the type shown in copending application Ser. No. 398,871, filed Sept. 19, 1973 now U.S. Pat. No. 3.909,571, in the name of Aumayer, entitled CONTACT STRUCTURE FOR HIGH VOLTAGE GAS BLAST CIRCUIT INTERRUPTER, and assigned to the assignee of the present invention. A high dielectric gas such as sulfur hexafluoride or other gases mixed with sulfur hexafluoride fills the bushing and communicates with the interior of the low-pressure main housing of the circuit breaker which receives the bushing. The gas is stationary within the bushing and circuit breaker housing and is at a low pressure of, for example, three atmospheres. The gas is supplied from a high-pressure container which will be at a pressure of greater than about fifteen atmospheres. The bottom of the conductive studs of the bushings are then electrically connected to the contacts of interrupter structures which contain blast valves to allow high pressure gas to blast through the contacts and into the low pressure housing when the contacts are operated.
The low-pressure gas system of two-pressure SF.sub.6 circuit breakers as pointed out above is at a pressure of about 45 p.s.i.g. at 70.degree.F. The saturation characteristic of SF.sub.6 is such that gas at 45 p.s.i.g. begins to liquify at temperatures slightly below -40.degree.F. Temperatures of -40.degree.F can be experienced, particularly in the more northerly regions of the North American Continent. The partial liquefaction of the gas will cause a reduction in the pressure in the lowpressure system, thereby reducing the dielectric insulation between the various circuit breaker parts and bushing parts. Moreover, the liquefaction in the bushing can give rise to possible contamination, which may cause dielectric tracking.
In the past and to overcome the problem of liquefaction of the low-pressure gas in the low-pressure portion of a two-pressure breaker, the normal engineering choice would be to lower the pressure in the two-pressure system to about 30 p.s.i.g. at 70.degree.F as contrasted to the nominal 45 p.s.i.g. at 70.degree.F. which is now used. The lowering of the gas pressure, however, would require larger clearances between parts and, therefore, is not economical.